Explicitly define layout of toolbar buttons

Before:

- Firefox's buttons looks OK
- Chrome (quirks mode): Buttons were aligned to the bottom (too much)
- Chrome (standards mode): Buttons were aligned to the top (too much)
- Opera/IE/Safari: Like Chrome (standards): Buttons too high

(Too high = Compare the other buttons to the rightmost button)

After:

- Firefox's button positions didn't change at all
- All buttons are aligned at the same level, across all browsers
Merge pull request #3022 from Rob--W/css-button-fix

Explicitly define layout of toolbar buttons
Implement progressive loading of PDFs